I think fiat and USD will stay whatever happen to crypto because it is important. When you buy crypto you need fiat or Usd. If you want to exchange your tokens before you can use and spend it you need to exchange it first into fiat money. Also fiat and Usd are centralized currency issued by the government while cryptocurrency is decentralized and intangible digital currency. It is difficult to say goodbye fiat and usd or just we say it may happen someday maybe it takes more years but i still doubt it to happen.